Cutting tool and round double sided cutting insert therefor

ABSTRACT

A cutting insert for retention in a rotating cutting tool having an axis of rotation has two opposing end surfaces and a peripheral surface extending therebetween, each end surface having a mutual first axis of symmetry passing through the end surfaces about which each end surface has N-fold rotational symmetry for some value of N where N is chosen from the group consisting of 2, 3 and 4. A peripheral cutting edge is formed at the junction between each end surface and the peripheral surface, the peripheral cutting edge having N curved cutting edges merging with N straight cutting edges which extend between the curved cutting edges at extremities thereof. The curved cutting edges of the two end surfaces are not aligned in an end view of the cutting insert along the first axis of symmetry.

FIELD OF THE INVENTION

The present invention relates to a milling cutting insert having roundedcutting edges and particularly to a double-sided cutting insert of sucha kind.

BACKGROUND OF THE INVENTION

When producing turbine blades, the final machining stage of the internalsurface of the turbine blade is typically done by means of millingcutting inserts having round cutting edges. In such a case, the totalusable sector used to cut by such a cutting insert is usually greaterthan 120°, therefore, the cutting insert may be indexed only two timessince a third indexing would not be able to utilize a full range sector,i.e., a cutting edge spanning more than 120°.

Known round cutting inserts or cutting inserts having round cuttingedges are often single sided, and, as mentioned, may fully utilize onlytwo cutting edges. For example, U.S. Pat. No. 4,175,896 discloses asingle sided cutting insert having two arcuate cutting edges which areeccentric to each other and separated from each other by diametricallysymmetric flat faces.

Round double sided cutting inserts are typically ceramic inserts and aredevoid of a central through bore for the passage of a clamping screw.The lack of a through bore is a disadvantage since it requires utilizinga more complex and expensive retaining system.

It is an object of the present invention to provide a double-sidedindexable cutting insert having four or more round cutting edges.

It is an object of the present invention to provide a double-sidedindexable cutting insert having four round cutting edges wherein each ofthe cutting edges extends more than 120°.

It is an object of the present invention to provide a tool holder for adouble sided cutting insert having four rounded cutting edges and athrough bore, the cutting insert being retained within the tool holderby means of a clamping screw that passes through the through bore.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a cuttinginsert for retention in a rotating cutting tool having an axis ofrotation, the cutting insert comprising: two opposing end surfaces and aperipheral surface extending therebetween, each end surface having amutual first axis of symmetry passing through the end surfaces aboutwhich each end surface has N-fold rotational symmetry for some value ofN where N is chosen from the group of 2, 3 and 4; a peripheral cuttingedge formed at the junction between each end surface and the peripheralsurface, the peripheral cutting edge comprising N curved cutting edgesmerging with N straight cutting edges which extend between the curvedcutting edges at extremities thereof; wherein: the curved cutting edgesof the two end surfaces are not aligned in an end view of the cuttinginsert along the first axis of symmetry.

Typically, N cutting edge axes, located in a mid portion of each of theN curved cutting edges, extend parallel to the first axis of symmetry,and each of the N curved cutting edges of one of the end surfaces isrotated about its associated cutting edge pivot axis relative to anopposite curved cutting edge of the other end surface, as seen in an endview of the cutting insert along the first axis of symmetry.

Advantageously, each of the N curved cutting edges lies on a torus.

Further advantageously, each operative curved cutting edge sweeps out aportion of a torus when the cutting insert is retained in the rotatingcutting tool and the rotating cutting tool is rotated about its axis ofrotation by 360°.

In one embodiment, one extremity of any given curved cutting edge islocated further from a median plane of the cutting insert than the otherextremity of the given curved cutting edge, the median plane beinglocated midway between the end surfaces.

In one embodiment, the curved cutting edges associated with one of theend surfaces are located in a first reference plane and the curvedcutting edges associated with the other end surface are located in asecond reference plane, the first and second reference planes beingparallel to each other and located equidistant from and on either sideof a median plane of the cutting insert, the median plane being locatedmidway between the end surfaces.

Typically, the cutting insert comprises a through bore, having a throughbore axis constituting the first axis of symmetry, that extends betweenthe two end surfaces; and the peripheral surface comprises a first,second, third and fourth pairs of side abutment surfaces, each pair ofside abutment surfaces has 180° rotational symmetry around the throughbore axis.

In one embodiment, the two end surfaces are identical.

If desired, a major portion of each of the curved cutting edges extendsalong an angle equal to or larger than 120° as seen along the first axisof symmetry.

In one embodiment, a first end surface of the two opposing end surfacesconstituting an upper surface defining a first reference plane, a secondend surface of the two opposing end surfaces constituting a lowersurface defining a second reference plane parallel to the firstreference plane; the curved cutting edges comprise a first main cuttingedge and a second main cutting edge; in an end view of the cuttinginsert the first main cutting edge has a first radius of curvature withrespect to a first cutting edge axis and the second main cutting edgehas a second radius of curvature with respect to a second cutting edgeaxis, the first cutting edge axis and the second cutting edge axis areparallel to the through bore axis and located at opposite sides thereof;the first cutting edge axis is located a first distance from the secondmain cutting edge, the through bore axis is located a second distancefrom the second main cutting edge, and the first distance is smallerthan the second distance.

Typically, the cutting insert has 180° rotational symmetry around asecond symmetry axis, the second symmetry axis lies on a median planebetween the first and the second reference planes, and intersects theperipheral surface at two median insert symmetry points; each of themedian insert symmetry points is formed at the intersection of a firstreference line with a second reference line, as seen in a first sideview of the cutting insert that is perpendicular to a given secondarycutting edge; the first reference line connects the leading end of afirst main cutting edge of a given end surface with the leading end of afirst main cutting edge of an opposite end surface; and the secondreference line connects the trailing end of a second main cutting edgeof a given end surface with the trailing end of a second main cuttingedge of an opposite end surface.

In one embodiment, the first cutting edge axis and the second cuttingedge axis are located at opposite sides of an imaginary plane that isperpendicular to a symmetry plane that contains the first axis ofsymmetry and the second symmetry axis.

Further in accordance with the present invention there is provided acutting insert for retention in a rotating cutting tool having an axisof rotation, the cutting insert comprising: two opposing end surfacesand a peripheral surface extending therebetween, each end surface havinga mutual first axis of symmetry passing through the end surfaces aboutwhich each end surface has 180° rotational symmetry; a peripheralcutting edge formed at the junction between a first end surface,constituting an upper surface, and the peripheral surface, theperipheral cutting edge comprising two curved cutting edges merging withtwo straight cutting edges which extend between the curved cutting edgesat extremities thereof; a continuously extending rake surface extendsinwardly from the peripheral cutting edge, the rake surface is slantedat a rake slant angle with respect to a second end surface, constitutinga lower surface; the upper surface comprises a rake inner extremity atthe innermost extremity of the rake surface and a bore upper end at theuppermost end of a through bore that extends between the end surfaces, alength between a given point on the peripheral cutting edge and the rakeinner extremity comprises a first rake length and a length between thegiven point and the bore upper end comprises a second rake length, thefirst rake length and the second rake length taken in a plane parallelto the lower surface; wherein: the rake slant angle is equal to orgreater than 25°; a rake extension ratio, defined as a ratio between thefirst rake length and the second rake length, is smaller than 1 andequal to or greater than 0.8; and a major portion of each of the curvedcutting edges lies on a torus and extends along an angle equal to orlarger than 120° as seen along the first axis of symmetry.

Typically, the peripheral surface comprises a first pair of sideabutment surfaces that converge towards each other in a direction towardthe upper surface, and, a third pair of side abutment surfaces thatconverge towards each other in a direction toward the upper surface.

Still further in accordance with the present invention there is provideda cutting tool having a longitudinal axis of rotation and comprising: atool body having at least one insert pocket formed in a front end of thetool body and a cutting insert retained in the at least one insertpocket, the at least one insert pocket comprises: a pocket base abutmentsurface; a threaded bore extending tangentially rearwardly from thepocket base abutment surface; pocket side walls extending upwardly fromthe pocket base abutment surface, two spaced apart of the pocket sidewalls are a first pocket abutment surface, forming an acute first pocketinternal angle with the pocket base abutment surface, and a secondpocket abutment surface, forming an acute second pocket internal anglewith the pocket base abutment surface.

In one embodiment, the cutting tool's cutting insert comprises: twoopposing end surfaces and a peripheral surface extending therebetween,each end surface having a mutual first axis of symmetry passing throughthe end surfaces about which each end surface has N-fold rotationalsymmetry, where N is a number from the group consisting of 2, 3 and 4; aperipheral cutting edge formed at the junction between each end surfaceand the peripheral surface, the peripheral cutting edge comprising Ncurved cutting edges (32, 34; 132, 134) merging with N straight cuttingedges which extend between the curved cutting edges at extremitiesthereof; wherein the curved cutting edges of the two end surfaces arenot aligned, as seen in an end view of the cutting insert.

The cutting tool's cutting insert may further comprise: a first, second,third and fourth pairs of side abutment surfaces, each pair of the sideabutment surfaces has 180° rotational symmetry around the through boreaxis, the first pair of side abutment surfaces converge towards eachother in a direction toward the upper surface, the second pair of sideabutment surfaces converge towards each other in a direction toward thelower surface, the third pair of side abutment surfaces converge towardseach other in a direction toward the upper surface, the fourth pair ofside abutment surfaces converge towards each other in a direction towardthe lower surface, the third pair has 180° rotational symmetry with thefourth pair around a second symmetry axis that passes between the thirdpair and the fourth pair, the upper surface defines a first referenceplane and the lower surface defines a second reference plane, the firstand second reference planes are parallel to a median plane that islocated midway between the upper surface and the lower surface, theperipheral surface forms with the first reference plane and with thesecond reference plane an obtuse first internal included angle, as seenin a first side view of the cutting insert that is perpendicular to agiven secondary cutting edge, the peripheral surface forms with thefirst reference plane and with the second reference plane an acutesecond internal included angle, as seen in a second side view of thecutting insert that is perpendicular to the first side view, the uppersurface is provided with a planar upper central abutment surface,constituting an insert upper base abutment surface, that extendsinwardly from an associated rake surface toward the through bore; thelower surface is provided with a planar lower central abutment surface,constituting an insert lower base abutment surface, that extendsinwardly from the associated rake surface toward the through bore;wherein: in a retained position of the cutting insert, the insert lowerbase abutment surface abuts the pocket base abutment surface, oneabutment surface of the insert first pair of side abutment surfacesabuts the first pocket abutment surface, one abutment surface of theinsert third pair of side abutment surfaces abuts the second pocketabutment surface, and, a clamping screw passes through the through boreof the cutting insert and threadingly engages the threaded bore of theinsert pocket.

In one embodiment, each side abutment surface of the first pair of sideabutment surfaces converges towards each other in a direction toward theupper surface; each side abutment surface of the second pair of sideabutment surfaces converges towards each other in a direction toward thelower surface; each side abutment surface of the third pair of sideabutment surfaces converges towards each other in a direction toward theupper surface; and each side abutment surface of the fourth pair of sideabutment surfaces converges towards each other in a direction toward thelower surface.

If desired, the third pair of side abutment surfaces has 180° rotationalsymmetry with the fourth pair of side abutment surfaces around thesymmetry axis.

Further if desired, the first pair of side abutment surfaces convergetowards each other in a direction toward the upper surface as viewed ina cross-section taken in a first section plane, the first section planecontains the through bore axis and is obliquely disposed, as seen in anend view of the cutting insert, with respect to a symmetry planecontaining the through bore axis and the symmetry axis.

Still further if desired, the second pair of side abutment surfacesconverge towards each other in a direction toward the lower surface asviewed in a cross-section taken in a second section plane, the secondsection plane contains the through bore axis and is obliquely disposedat a plane angle, as seen in a top view of the cutting insert, withrespect to the symmetry plane and with respect to the first sectionplane.

In one embodiment, the plane angle is 80°.

Typically, the peripheral surface forms with the first reference planeand with the second reference plane an obtuse first internal includedangle, as seen in a first side view of the cutting insert that isperpendicular to a secondary cutting edge; and the peripheral surfaceforms with the first reference plane and with the second reference planean acute second internal included angle, as seen in a second side viewof the cutting insert that is perpendicular to the first side view.

If desired, the first main cutting edge and the second main cutting edgefollow a major first radius of curvature along the major portion thereofand a minor second radius of curvature along a minor portion thereof.

Typically, the at least one insert pocket is provided with a pocketabutment relief surface that is located above the second pocket abutmentsurface, the pocket abutment relief surface being relieved from theadjacent abutment surface of the insert fourth pair of side abutmentsurfaces in a retained position of the cutting insert.

In one embodiment, the cutting insert is four times indexable within theat least one insert pocket.

Typically, the first pocket abutment surface forms with the secondpocket abutment surface an acute pocket angle as seen in a top view ofthe at least one insert pocket.

If desired, the at least one insert pocket is provided with a pocketrelief channel located between the pocket side walls and the pocket baseabutment surface.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how thesame may be carried out in practice, reference will now be made to theaccompanying drawings, in which:

FIG. 1 is a perspective view of a cutting tool in accordance with thepresent invention;

FIG. 2 is shows the cutting tool of FIG. 1 with a cutting insert inaccordance with the present invention removed from its pocket;

FIG. 3 is an enlarged perspective view of the cutting insert of FIG. 2;

FIG. 4 is an end view of the cutting insert of FIG. 3;

FIG. 5 is an end view of the cutting insert of FIG. 3 showing thecutting edges of both end surfaces without showing the through bore;

FIG. 6 is a cross-sectional view of the cutting insert of FIG. 3 takenalong line VI-VI in FIG. 7;

FIG. 7 is a first side view of the cutting insert of FIG. 3;

FIG. 8 is a second side view of the cutting insert of FIG. 3;

FIG. 9 is a cross-sectional view of the cutting insert of FIG. 3 takenalong line IX-IX in FIG. 4;

FIG. 10 is a cross-sectional view of the cutting insert of FIG. 3 takenalong line X-X in FIG. 4;

FIG. 11 is a top view of the insert pocket of FIG. 2;

FIG. 12 is a cross-sectional view of the insert pocket taken along lineXII-XII in FIG. 11;

FIG. 13 is a cross-sectional view of the insert pocket taken along lineXII-XII in FIG. 11 with the cutting insert retained in the insert pocketby a clamping screw;

FIG. 14 is a top view of the cutting insert mounted in the insert pocketand machining a workpiece;

FIG. 15 is a perspective view of another embodiment of the cuttinginsert in accordance with the present invention;

FIG. 16 is a first side view of the cutting insert of FIG. 15;

FIG. 17 is a second side view of the cutting insert of FIG. 15;

FIG. 18 is a top view of the cutting insert of FIG. 15;

FIG. 19 is a cross-sectional view of the cutting insert of FIG. 15 takenalong line XIX-XIX in FIG. 18;

FIG. 20 is a cross-sectional view of the cutting insert of FIG. 15 takenalong line XX-XX in FIG. 18;

FIG. 21 is a cross-sectional view of the cutting insert of FIG. 15 takenalong line XXI-XXI in FIG. 19; and

FIG. 22 is a cross-sectional view of another embodiment of the cuttinginsert.

DETAILED DESCRIPTION OF THE INVENTION

Attention is first drawn to FIGS. 1 and 2 showing a cutting tool 10 inaccordance with the present invention. The cutting tool 10 has alongitudinal axis of rotation A defining a front-to-rear direction ofthe cutting tool 10 and a direction of rotation R. The cutting tool 10comprises a tool body 12 having a plurality of insert pockets 14 formedin a front end 16 of the tool body 12. A cutting insert 18 is retainedin each of the insert pockets 14 by means of a clamping screw 20. Thecutting insert 18 may be preferably made from cemented carbide powdersby pressing and sintering or by injection molding techniques.

Attention is now drawn to FIGS. 3 to 10. The cutting insert 18 comprisesan upper surface 22, defining a first reference plane P1, a lowersurface 24, defining a second reference plane P2, and a peripheralsurface 26 extending between the upper surface 22 and the lower surface24. The upper surface 22 and the lower surface 24 constitute endsurfaces 28 of the cutting insert 18. In some embodiments, the endsurfaces 28 may be identical, and the first reference plane P1 may beparallel to the second reference plane P2.

The cutting insert 18 is provided with a through bore 30 having athrough bore axis B. The through bore 30 extends between the uppersurface 22 and the lower surface 24. In some embodiments, the cuttinginsert 18 may have 180° rotational symmetry around the through bore axisB.

In some embodiments, each of the end surfaces 28 may comprise a firstmain cutting edge 32 and a second main cutting edge 34. The first maincutting edge 32 and the second main cutting edge 34 may be identical.The cutting edges are formed at the junction between each end surface 28and the peripheral surface 26.

FIG. 5 is an end view of the cutting insert 18 showing the cutting edgesof both end surfaces without showing the through bore 30. The cuttingedges of the upper surface 22 are shown in solid lines and the cuttingedges of the lower surface 24 are shown in dashed lines. As can be seen,the first and second main cutting edges 32, 34 of the upper surface 22are angularly shifted with respect to the first and second main cuttingedges 32′, 34′ of the lower surface 24. Thus, as seen in an end view ofthe cutting insert 18, the main cutting edges of a given end surface 28are not aligned with the main cutting edges of the opposite end surface28.

For sake of clarity, the non-alignment of the main cutting edges may bedescribed in the following manner. Each of the cutting edges 32, 34 hasa cutting edge pivot axis C associated therewith. The cutting edge pivotaxis C is located in a mid portion 35 of the cutting edge and extendsparallel to an axis of symmetry S which will be later described. The midportion 35 refers to a region including the geometrical center of thecurved cutting edge and not necessarily to the actual geometrical centerof the curved cutting edge.

Thus, as can be seen in FIG. 5, each of the curved cutting edges 32, 34of one end surface 28 is rotated about its associated cutting edge pivotaxis C relative to the opposite curved cutting edge 34′, 32′ of theother end surface 28. Thus, though the curved cutting edges 32, 34intersect cutting edges 34′, 32′, respectively, when viewed along axisB, they clearly are not aligned.

In an end view of the cutting insert 18, the first main cutting edge 32has a first radius of curvature R1 with respect to a first cutting edgeaxis A1, and, the second main cutting edge 34 has a second radius ofcurvature R2 with respect to a second cutting edge axis A2. As shown inFIG. 5, the first cutting edge axis A1 and the second cutting edge axisA2 may be parallel to the through bore axis B and located at oppositesides thereof. As shown, the first cutting edge axis A1 is located afirst distance D1 from the second main cutting edge 34, the through boreaxis B is located a second distance D2 from the second main cutting edge34, and the first distance D1 is smaller than the second distance D2.

Hence, as seen in an end view of the cutting insert 18, the firstcutting edge axis A1 is located closer to the second main cutting edge34 than the through bore axis B. In a similar manner, the second cuttingedge axis A2 is located closer to the first main cutting edge 32 thanthe through bore axis B.

The first main cutting edge 32 and the second main cutting edge 34 donot have to follow a radius of curvature, and they may be curved inother forms. For example, in one embodiment that is shown in dashedlines in the upper portion of FIG. 5, the first main cutting edge 32 andthe second main cutting edge 34 may follow a major first radius ofcurvature MR11 along a major angular portion MP11 thereof and a minorsecond radius of curvature MR21 along a minor angular portion MP21thereof.

In that embodiment, the major first radius of curvature MR11 may bedifferent than the minor second radius of curvature MR21. Furthermore,the major first radius of curvature MR11 may extend along a relativelylarge angle, represented by the major portion MP11, for example, 120°,wherein the minor second radius of curvature MR21 may extend along asmaller angle, represented by the minor portion MP21, for example, 20°.This embodiment is shown with respect to only one main cutting edge, inthis case, with respect to the second main cutting edge. However, theembodiment may be equally applicable to the first and second maincutting edges.

In another embodiment that is shown in dashed lines in the lower portionof FIG. 5, the major first radius of curvature MR12 may extend along arelatively large angular portion MP12, for example, 140°, wherein theminor second radius of curvature MR22 may extend along a much smallerangular portion MP22, for example, 1° to 10°. This embodiment is shownwith respect to only one main cutting edge, in this case, with respectto the first main cutting edge. However, the embodiment may be equallyapplicable to the first and second main cutting edges.

In other embodiments, the first main cutting edge 32 and the second maincutting edge 34 are formed from several sections (not shown in thefigures) that have different radii of curvature and merge with eachother to form a continuously curved main cutting edge.

The first main cutting edge 32 has a leading end 36 and a trailing end38. The second main cutting edge 34 has a leading end 40 and a trailingend 42. A first secondary cutting edge 44 merges, at a leading end 46thereof, with the leading end 36 of the first main cutting edge 32, and,at a trailing end 48 thereof, with the trailing end 42 of the secondmain cutting edge 34.

A second secondary cutting edge 50 merges, at a leading end 52 thereof,with the leading end 40 of the second main cutting edge 34, and, at atrailing end 54 thereof, with the trailing end 38 of the first maincutting edge 32.

In one embodiment, the first secondary cutting edge 44 is identical tothe second secondary cutting edge 50. The first and second secondarycutting edges 44, 50 are mainly used for performing ramp-down operationsand their length and shape are determined according to machining needs.In one embodiment, the first secondary cutting edge 44 and the secondsecondary cutting edge 50 are formed along straight lines.

As can be seen in FIG. 5, in addition to the fact that the first andsecond main cutting edges 32, 34 of the upper surface 22 are angularlyshifted with respect to the respective second and first main cuttingedges 34′, 32′ of the lower surface 24, the first and second secondarycutting edges 44, 50 of the upper surface 22 are linearly shifted withrespect to the first and second secondary cutting edges 44′, 50′ of thelower surface 24.

The cutting insert 18 may have 180° rotational symmetry around asymmetry axis S. The symmetry axis S lies on a median plane M betweenthe first and second reference planes P1, P2, and intersects theperipheral surface 26 at two median insert symmetry points 56. Each ofthe median insert symmetry points 56 is formed at the intersection of afirst reference line L1 with a second reference line L2, as seen in FIG.7. FIG. 7 being a first side view of the cutting insert 18 that isperpendicular to the first or the second secondary cutting edges 44, 50.

In the embodiment described above, the main cutting edges 32, 34 areidentical to each other, the secondary cutting edges 44, 50 areidentical to each other, and both end surfaces 28, namely, the uppersurface 22 and the lower surface 24 are identical to each other.Therefore, for numbering the cutting edges of the lower surface 24, anarbitrary decision was made to rotate the cutting insert 18 180° aroundthe symmetry axis S. In this position, the cutting edges which werepreviously located in the upper surface 22 are now located in the lowersurface 24 and a prime sign was added to their number. Thus, forexample, the corresponding cutting edge of the first main cutting edge32 is marked 32′, and so on.

The first reference line L1 connects the leading end 36 of a first maincutting edge 32 of a given end surface 28 with the leading end 36′ of afirst main cutting edge 32′ of the opposite end surface 28. The secondreference line L2 connects the trailing end 42 of a second main cuttingedge 34 of a given end surface 28 with the trailing end 42′ of a secondmain cutting edge 34′ of the opposite end surface 28.

The peripheral surface 26 comprises a first pair of side abutmentsurfaces 58, a second pair of side abutment surfaces 60, a third pair ofside abutment surfaces 62 and a fourth pair of side abutment surfaces64. In one embodiment, each pair of the side abutment surfaces 58, 60,62, 64 has 180° rotational symmetry around the through bore axis B.

As seen in FIG. 9, each side abutment surface of the first pair of sideabutment surfaces 58 converges towards each other in a direction towardthe upper surface 22. FIG. 9 is a view of a cross-section taken in afirst section plane P3. As seen in the end view of FIG. 4, the firstsection plane P3 contains the through bore axis B and is obliquelydisposed with respect to a symmetry plane SP containing the through boreaxis B and the symmetry axis S.

As seen in FIG. 10, each side abutment surface of the second pair ofside abutment surfaces 60 converges towards each other in a directiontoward the lower surface 24. FIG. 10 is a view of a cross-section takenin a second section plane P4. The second section plane P4 contains thethrough bore axis B and is obliquely disposed, as seen in FIG. 4, withrespect to the symmetry plane SP and with respect to the first sectionplane P3. In one embodiment, the first section plane P3 forms with thesecond section plane P4 a plane angle φ of 80°, with a supplementaryangle of 100°.

The plane angle φ between the first section plane P3 and the secondsection plane P4 may be seen also in FIG. 6, where the first sectionplane P3 is perpendicular to the first pair of side abutment surfaces 58and the second section plane P4 is perpendicular to the second pair ofside abutment surfaces 60. Both first section plane P3 and secondsection plane P4 are perpendicular to the median plane M, which alsocontains the symmetry axis S.

As seen in FIG. 8, each side abutment surface of the third pair of sideabutment surfaces 62 converges towards each other in a direction towardthe median plane M. Similarly, each side abutment surface of the fourthpair of side abutment surfaces 64 also converges towards each other in adirection toward the median plane M. One may likewise consider the thirdpair of side abutment surfaces 62 to converge towards each other in adirection toward the upper surface 22, and the fourth pair of sideabutment surfaces 64 to converge towards each other in a directiontoward the lower surface 24.

In one embodiment, the third pair of side abutment surfaces 62 has 180°rotational symmetry with the fourth pair of side abutment surfaces 64around the symmetry axis S.

As seen in FIG. 7, the peripheral surface 26 forms with the firstreference plane P1 and with the second reference plane P2 an obtusefirst internal included angle α. FIG. 7, being a first side view of thecutting insert 18, is viewed from a direction perpendicular to asecondary cutting edge 44.

FIG. 8 is a second side view of the cutting insert 18 taken in adirection perpendicular to the symmetry plane SP. The directionperpendicular to the symmetry plane SP is represented by an imaginaryplane N. Hence, the direction of the second side view of the cuttinginsert 18 is perpendicular to the direction of the first side view ofthe cutting insert 18. As seen in FIG. 8, the peripheral surface 26forms with the first reference plane P1 and with the second referenceplane P2 an acute second internal included angle β.

As can be seen in FIG. 5, the first cutting edge axis A1 and the secondcutting edge axis A2 are located at opposite sides of the symmetry planeSP. The first cutting edge axis A1 is distanced a first axis distance D5from the symmetry plane SP and the second cutting edge axis A2 isdistanced a second axis distance D6 from the symmetry plane SP.According to one embodiment, the first axis distance D5 is equal to thesecond axis distance D6.

In one embodiment, as can be seen in FIG. 5, the first cutting edge axisA1 and the second cutting edge axis A2 may be located at opposite sidesof the imaginary plane N. In that case, the first cutting edge axis A1is distanced a third axis distance D7 from the imaginary plane N and thesecond cutting edge axis A2 is distanced a fourth axis distance D8 fromthe imaginary plane N. According to one embodiment, the third axisdistance D7 is equal to the fourth axis distance D8.

As can be best seen in FIGS. 3 and 4, the first and second main cuttingedges 32, 34 of a given end surface 28 and their associated first andsecond secondary cutting edges 44, 50 form a continuously extendingcutting edge 66 that is associated with a rake surface 68. In oneembodiment, the rake surface 68 extends continuously along the entirelength of the cutting edge 66. The rake surface 68 extends inwardly fromthe cutting edge 66 toward the through bore axis B, and slopesrearwardly toward the median plane M in the direction of the other endsurface 28.

The upper surface 22 of the cutting insert 18 is provided with a planarupper central abutment surface 70 that extends inwardly from theassociated rake surface 68 toward the through bore 30. Similarly, thelower surface 24 is provided with a planar lower central abutmentsurface 72 that extends inwardly from the associated rake surface 68toward the through bore 30.

As seen in FIG. 9, the upper central abutment surface 70 is distanced athird distance D3 from the lower central abutment surface 72, the firstreference plane P1 is distanced a fourth distance D4 from the secondreference plane P2, and, the third distance D3 is smaller than thefourth distance D4. Thus, the central abutment surfaces 70, 72 arerecessed toward the median plane M, relative to their respectiveassociated reference planes P1, P2.

Attention is now drawn to FIGS. 11 to 14. Each insert pocket 14comprises a pocket base abutment surface 74. The pocket base abutmentsurface 74 may be planar, and it may be formed as a single surface, asshown in FIG. 11, or be divided into several surfaces. If the pocketbase abutment surface is divided into several surfaces, the severalsurfaces may be separated by relief grooves. A threaded bore 76 extendstangentially rearwardly from the pocket base abutment surface 74.

The insert pocket 14 further comprises pocket side walls 78 that extendupwardly from the pocket base abutment surface 74. Two of the pocketside walls 78 form pocket abutment surfaces. In one embodiment, thepocket abutment surfaces are spaced apart by a pocket side wall 78 thatdoes not form a pocket abutment surface.

The pocket abutment surfaces comprise a first pocket abutment surface 80and a second pocket abutment surface 82. The first pocket abutmentsurface 80 forms an acute first pocket internal angle γ with the pocketbase abutment surface 74, and the second pocket abutment surface 82forms an acute second pocket internal angle δ with the pocket baseabutment surface 74.

In a retained position of the cutting insert 18, the insert lowercentral abutment surface 72 abuts the pocket base abutment surface 74,one abutment surface of the insert first pair of side abutment surfaces58 abuts the first pocket abutment surface 80, one abutment surface ofthe insert third pair of side abutment surfaces 62 abuts the secondpocket abutment surface 82, and, the clamping screw 20 passes throughthe through bore 30 of the cutting insert 18 and threadingly engages thethreaded bore 76 of the insert pocket 14.

By means of the acute internal angles γ and δ, the seating of the insertfirst 58 and third 62 operative side abutment surfaces against the firstand second pocket abutment surfaces 80, 82 provides a firm clamping ofthe cutting insert 18 in a dove-tail manner thus better securing thecutting insert 18 within the insert pocket 14.

The insert pocket 14 is provided with a pocket abutment relief surface84 that is located above the second pocket abutment surface 82 and awayfrom the pocket base abutment surface 74. In a retained position of thecutting insert 18, the pocket abutment relief surface 84 is relievedfrom the adjacent abutment surface of the insert fourth pair of sideabutment surfaces 64.

The insert pocket 14 is further provided with a pocket relief channel86. The pocket relief channel 86 is located between the pocket sidewalls 78 and the pocket base abutment surface 74. When the cuttinginsert 18 is retained within the insert pocket 14, the pocket reliefchannel 86 provides adequate clearance to the cutting edge 66 associatedwith the lower central abutment surface 72 of the cutting insert 18 thatabuts the pocket base abutment surface 74.

As can be best seen in FIG. 11, the first pocket abutment surface 80forms with the second pocket abutment surface 82 an acute pocket angle θin order to adequately support the side abutment surfaces of the cuttinginsert 18.

FIG. 14 shows a top view of the cutting insert 18 when retained withinan insert pocket 14 and machining a workpiece W. For sake of clarity,only a part of the tool body 12 is shown. As shown, the active maincutting edge 32 that may be round along a relatively large arc enablesthe cutting tool 10 to effectively machine, for example, internalprofiles of turbine blades T along the entire internal surface 88 of theblade T.

The cutting insert 18 drawn in solid lines shows the location of thecutting insert when beginning to machine the internal surface 88 of theturbine blade T. The cutting insert 18 drawn in dashed lines shows thelocation of the cutting insert when ending the machining of the internalsurface 88 of the turbine blade T.

As can be seen by the two extreme positions of the cutting insert 18with respect to the internal surface 88 of the turbine blade T, theentire active main cutting edge 32 is operative during this machiningprocess. Since the main cutting edge 32 extends along a relatively largearc, it is effective for machining an entire concave internal surface 88of a turbine blade T.

In other applications (not shown), the active secondary cutting edge 44may effectively perform ramp-down operations.

Hence, as described above, according to one embodiment of the presentinvention, the cutting insert 18 is capable of performing a variety ofcutting operations, and may be four times indexable within an insertpocket 14.

Attention is now drawn to FIGS. 15 to 21 which show another embodimentof the cutting insert. In these figures, like parts are designated withlike reference numerals with the addition of 100. The cutting insert 118shown in FIGS. 15 to 21 has a structure similar to the structure of thecutting insert 18 described above, however, it differs in the shape ofits cutting edges.

The cutting insert 118 comprises, in each end surface 128 thereof, firstand second main cutting edges 132, 134 that are connected, throughextremities thereof, to first and second secondary cutting edges 144,150.

The first main cutting edge 132 has a leading end 136 and a trailing end138. The second main cutting edge 134 has a leading end 140 and atrailing end 142. A first secondary cutting edge 144 merges, at aleading end 146 thereof, with the leading end 136 of the first maincutting edge 132, and, at a trailing end 148 thereof, with the trailingend 142 of the second main cutting edge 134.

A second secondary cutting edge 150 merges, at a leading end 152thereof, with the leading end 140 of the second main cutting edge 134,and, at a trailing end 154 thereof, with the trailing end 138 of thefirst main cutting edge 132.

In one embodiment, the first secondary cutting edge 144 may be identicalto the second secondary cutting edge 150. The first and second secondarycutting edges 144, 150 are mainly used for performing ramp-downoperations and their length and shape are determined according tomachining needs. In one embodiment, the first main cutting edge 132 maybe identical to the second main cutting edge 134.

As can be clearly seen in the figures, the leading end 136 of the firstmain cutting edge 132 is located further from the median plane M thanthe trailing end 138 of the first main cutting edge. Similarly, theleading end 140 of the second main cutting edge 134 is located furtherfrom the median plane M than the trailing end 142 of the second maincutting edge 134. Such a construction of the first and second maincutting edges 132, 134 provides the cutting insert 118 a highly positiverake with respect to the upper central abutment surface 170.

Each of the first and second main cutting edges 132, 134 lies on atorus. Furthermore, when the cutting insert is retained in the insertpocket 14 of the cutting tool 10, and the cutting tool is rotated aboutits axis of rotation A by 360°, each point on an operative main cuttingedge, i.e., first main cutting edge 132 or second main cutting edge 134,sweeps out a portion of a torus.

The first and second main cutting edges 132, 134 and their associatedfirst and second secondary cutting edges 144, 150 form a continuouslyextending cutting edge 166 that is associated with a rake surface 168that may extend continuously along the entire circumference of an endsurface 128. As shown in FIGS. 18 to 21, the rake surface 168 mergeswith the upper central abutment surface 170 through an undercut 90 whichserves two purposes. First, it enables better control of the chipsproduced during machining. Second, it enables better distinguishing ofthe upper central abutment surface 170 with respect to the upper surface122 of the cutting insert 118, thus assuring satisfactory abutmentproperties of the upper central abutment surface 170. As can be seen inFIGS. 19 to 21, the undercut 90 may vary in size and shape at differentcross-sections of the cutting insert 118. Similarly, the same may beapplied to the lower surface 124 of the cutting insert 118.

In the embodiments described above, the height orientation of the maincutting edges 132, 134 with respect to the central abutment surface 170is such that the cutting edges are higher than the central abutmentsurface, i.e., the central abutment surface 170 is located closer to themedian plane M than the main cutting edges 132, 134. However, in otherembodiments (not shown) the main cutting edges 132, 134, or at least aportion thereof, may be located closer to the median plane M than thecentral abutment surface 170.

The construction of the cutting insert 118 enables considerableadvantages during machining, as can be appreciated by a person skilledin the art. The highly positive rake of the main cutting edges providesthe operative main cutting edge of the cutting insert 118, when thecutting insert 118 is mounted in the cutting tool 10 during machining,with a less negative rake angle, compared to the negative rake angle ofthe cutting insert 18 shown in FIG. 1 that is mounted in a negativeaxial positioning. The less negative axial rake leads to easier cutting,better chip removal, lower cutting forces, and, lower power consumption.This construction of the cutting insert 118 may be used when it isrequired to machine relatively high tensile materials and hightemperature alloys. A further advantage in this case may be avoidingadhesion of the chips.

Attention is now drawn to FIG. 22 which shows another embodiment of thecutting insert. In this figure, like parts are designated with likereference numerals with the addition of 200.

As shown, a cutting insert 218 comprises two end surfaces 228 and aperipheral surface 226 extending therebetween. One of the end surfaces228 forms an upper surface 222 similar to the upper surface 122 of thecutting insert 118 described above, and, another end surface forms alower surface 224 of the cutting insert 218. The upper surface 222comprises a continuously extending peripheral cutting edge 266associated with a continuously extending rake surface 268.

The peripheral cutting edge 266 comprises two curved cutting edges,namely, a first main cutting edge 232 and a second main cutting edge234, and, two straight cutting edges, namely, a first secondary cuttingedge and a second secondary cutting edge 250 connecting betweenextremities of the first and second main cutting edges 232, 234 (thefirst secondary cutting edge is not shown).

In order to better cut ductile materials, the rake surface 268 is formedconsiderably larger than the rake surface 168 of the cutting insert 118.The rake surface 268 is slanted at a rake slant angle λ with respect tothe lower surface 224 of the cutting insert 218. The rake slant angle λis relatively large and is preferably equal to or greater than 25°.

A rake inner extremity 92 is defined at a region where an innermostextremity of the rake surface 268 merges with a bore peripheral region94 that surrounds the through bore 30. The bore peripheral region 94merges with the through bore 30 at a bore upper end 96 and it extendsgenerally parallel to the lower surface 224 of the cutting insert 218. Afirst rake length H1 is defined between a given point 98 on theperipheral cutting edge 266 and the rake inner extremity 92, measured ina plane parallel to the lower surface 224. A second rake length H2 isdefined between the same given point 98 on the peripheral cutting edge266 and the bore upper end 96, measured in a plane parallel to the lowersurface 224.

A rake extension ratio E is defined as a ratio between the first rakelength H1 and the second rake length H2. The rake extension ratio E mayvary within a preferable range. According to one embodiment, the rakeextension ratio E is smaller than b 1 and equal to or greater than 0.8.

Since the rake surface 268 is relatively very large, it extends, in aninward direction of the cutting insert 218, almost to the through bore30. With such a construction, the upper surface 222 lacks a centralabutment surface, in contrary to the existence of the central abutmentsurface 170 in the upper surface 122 of the cutting insert 118.

Thus, since the cutting insert 218 lacks a central abutment surface, itcannot be used as a double sided cutting insert. Therefore, the lowersurface 224 of the cutting insert 118 lacks cutting edges, and is formedflat in order to serve solely as an abutment surface. Accordingly, theperipheral surface 226 is formed only with the abutment surfacesrequired for abutment of a single sided cutting insert, namely, thefirst pair of side abutment surfaces 258, and, the third pair of sideabutment surfaces 262 (not shown).

Although the present invention has been described to a certain degree ofparticularity, it should be understood that various alterations andmodifications could be made without departing from the spirit or scopeof the invention as hereinafter claimed.

The cutting insert is not limited to have two main cutting edges on eachend surface. In one embodiment (not shown), the cutting insert isprovided with three main cutting edges which are connected, atextremities thereof, by three secondary cutting edges. The three maincutting edges may be identical. Likewise, the three secondary cuttingedges may be identical.

In one embodiment (not shown), the cutting insert is provided with fourmain cutting edges which are connected, at extremities thereof, by foursecondary cutting edges. The four main cutting edges may be identical.Likewise, the four secondary cutting edges may be identical.

Thus, a cutting insert according to the present invention may beretained by a clamping screw passing through a through bore. The cuttinginsert may have four, six or eight rounded cutting edges, wherein thecutting insert may be indexed four, six or eight times. The cuttingedges may extend along a large arc and may extend at an angle largerthan 120°.

What is claimed is:
 1. A double-sided cutting insert (18, 118) forretention in a rotating cutting tool having an axis of rotation, thecutting insert comprising: two opposing end surfaces (28, 128) and aperipheral surface (26) extending therebetween, each end surface havinga mutual first axis of symmetry (B) passing through the end surfacesabout which each end surface has N-fold rotational symmetry, where N isa number from the group consisting of 2, 3 and 4; a peripheral cuttingedge (66, 166) formed at the junction between each end surface and theperipheral surface, the peripheral cutting edge comprising N curvedcutting edges (32, 34; 132, 134) merging with N straight cutting edges(44, 50; 144, 150) which extend between the curved cutting edges atextremities thereof; wherein: the curved cutting edges of the two endsurfaces are not aligned, as seen in an end view of the cutting insert:and each of the N curved cutting edges (32, 34; 132, 134) lies on atorus and extends along an angle equal to or larger than 120°, as seenalong the first axis of symmetry (B).
 2. The cutting insert (18, 118)according to claim 1, wherein: N cutting edge pivot axes (C), located ina mid portion (35) of each of the N curved cutting edges, extendparallel to the first axis of symmetry (B), and each of the N curvedcutting edges (32, 34; 132, 134) of one of the end surfaces (28, 128) isrotated about its associated cutting edge pivot axis (C) relative to anopposite curved cutting edge (34′, 32′) of the other end surface, asseen in an end view of the cutting insert.
 3. A double-sided cuttinginsert (18, 118) for retention in a rotating cutting tool having an axisof rotation, the cutting insert comprising: two opposing end surfaces(28, 128) and a peripheral surface (26) extending therebetween, each endsurface having a mutual first axis of symmetry (B) passing through theend surfaces about which each end surface has N-fold rotationalsymmetry, where N is a number from the group consisting of 2, 3 and 4; aperipheral cutting edge (66, 166) formed at the junction between eachend surface and the peripheral surface, the peripheral cutting edgecomprising N curved cutting edges (32, 34; 132, 134) merging with Nstraight cutting edges (44, 50; 144, 150) which extend between thecurved cutting edges at extremities thereof; wherein: the curved cuttingedges of the two end surfaces are not aligned, as seen in an end view ofthe cutting insert; the two end surfaces (28, 128) are identical; amajor angular portion (MP11, MP12) of each of the curved cutting edgesextends along an angle equal to or larger than 120° as seen in an endview of the cutting insert.
 4. The cutting insert (118) according toclaim 1, wherein: one extremity (136, 140) of any given curved cuttingedge (132, 134) is located further from a median plane (M) of thecutting insert than the other extremity (138, 142) of the given curvedcutting edge, the median plane being located midway between the endsurfaces.
 5. A double-sided cutting insert (18, 118) for retention in arotating cutting tool having an axis of rotation, the cutting insertcomprising: two opposing end surfaces (28, 128) and a peripheral surface(26) extending therebetween, each end surface having a mutual first axisof symmetry (B) passing through the end surfaces about which each endsurface has N-fold rotational symmetry, where N is a number from thegroup consisting of 2, 3 and 4; a peripheral cutting edge (66, 166)formed at the junction between each end surface and the peripheralsurface, the peripheral cutting edge comprising N curved cutting edges(32, 34; 132, 134) merging with N straight cutting edges (44, 50; 144,150) which extend between the curved cutting edges at extremitiesthereof; wherein: the curved cutting edges of the two end surfaces arenot aligned, as seen in an end view of the cutting insert; and each ofthe N curved cutting edges (32, 34; 132, 134) extends along an angleequal to or larger than 120°, as seen along the first axis of symmetry(B).
 6. The cutting insert (18) according to claim 1, wherein: thecurved cutting edges (32, 34) associated with one of the end surfaces(28) are located in a first reference plane (P1) and the curved cuttingedges (32′, 34′) associated with the other end surface are located in asecond reference plane (P2), the first and second reference planes beingparallel to each other and located equidistant from and on either sideof a median plane (M) of the cutting insert, the median plane beinglocated midway between the end surfaces.
 7. The cutting insert (18, 118)according to claim 1, wherein: the two end surfaces (28, 128) areidentical.
 8. The cutting insert according to claim 7, wherein: a majorangular portion (MP11, MP12) of each of the curved cutting edges extendsalong an angle equal to or larger than 120° as seen in an end view ofthe cutting insert.
 9. The cutting insert (18, 118) according to claim8, wherein: the first main cutting edge (32, 132) and the second maincutting edge (34, 134) follow a major first radius of curvature (MR11,MR12) along the major angular portion (MP11, MP12) thereof and a minorsecond radius of curvature (MR21, MR22) along a minor angular portion(MP21, MP22) thereof.
 10. A double-sided cutting insert (18, 118) forretention in a rotating cutting tool having an axis of rotation, thecutting insert comprising: two opposing end surfaces (28, 128) and aperipheral surface (26) extending therebetween, each end surface havinga mutual first axis of symmetry (B) passing through the end surfacesabout which each end surface has N-fold rotational symmetry, where N isa number from the group consisting of 2, 3 and 4 2 and 4; a peripheralcutting edge (66, 166) formed at the junction between each end surfaceand the peripheral surface, the peripheral cutting edge comprising Ncurved cutting edges (32, 34; 132, 134) merging with N straight cuttingedges (44, 50; 144, 150) which extend between the curved cutting edgesat extremities thereof; wherein: the curved cutting edges of the two endsurfaces are not aligned, as seen in an end view of the cutting insert;the cutting insert comprises a through bore (30), having a through boreaxis (B) constituting the first axis of symmetry, that extends betweenthe two end surfaces (28); and the peripheral surface (26) comprisesfirst (58), second (60), third (62) and fourth (64) pairs of sideabutment surfaces, each pair of side abutment surfaces having 180°rotational symmetry around the through bore axis (B).
 11. The cuttinginsert according to claim 10, wherein: a first end surface of the twoopposing end surfaces (28, 128) constituting an upper surface (22, 122)defining a first reference plane (P1), a second end surface of the twoopposing end surfaces constituting a lower surface (24, 124) defining asecond reference plane (P2) parallel to the first reference plane; thecurved cutting edges comprise a first main cutting edge (32, 132) and asecond main cutting edge (34, 134); in an end view of the cutting insertthe first main cutting edge (32, 132) has a first radius of curvature(R1) with respect to a first cutting edge axis (A1) and the second maincutting edge (34, 134) has a second radius of curvature (R2) withrespect to a second cutting edge axis (A2), the first cutting edge axis(A1) and the second cutting edge axis (A2) are parallel to the throughbore axis (B) and located at opposite sides thereof; the first cuttingedge axis (A1) is located a first distance (D1) from the second maincutting edge (34, 134), the through bore axis (B) is located a seconddistance (D2) from the second main cutting edge (34, 134), and the firstdistance (D1) is smaller than the second distance (D2).
 12. The cuttinginsert (18, 118) according to claim 11, wherein: the cutting insert has180° rotational symmetry around a second symmetry axis (S), the secondsymmetry axis lies on a median plane (M) between the first and thesecond reference planes (P1, P2), and intersects the peripheral surface(26) at two median insert symmetry points (56); each of the medianinsert symmetry points is formed at the intersection of a firstreference line (L1) with a second reference line (L2), as seen in afirst side view of the cutting insert that is perpendicular to a givensecondary cutting edge; the first reference line (L1) connects theleading end (36, 136) of a first main cutting edge (32, 132) of a givenend surface (28, 128) with the leading end (36, 136) of a first maincutting edge (32, 132) of an opposite end surface (28, 128); and thesecond reference line (L2) connects the trailing end (42, 142) of asecond main cutting edge (34, 134) of a given end surface (28) with thetrailing end (42, 142) of a second main cutting edge (34, 134) of anopposite end surface (28).
 13. The cutting insert (18, 118) according toclaim 12, wherein the first cutting edge axis (A1) and the secondcutting edge axis (A2) are located at opposite sides of an imaginaryplane (N) that is perpendicular to a symmetry plane (SP) that containsthe first axis of symmetry (B) and the second symmetry axis (S).
 14. Thecutting insert (18,118) according to claim 13, wherein: each sideabutment surface of the first pair of side abutment surfaces (58)converges towards each other in a direction toward the upper surface(22, 122); each side abutment surface of the second pair of sideabutment surfaces (60) converges towards each other in a directiontoward the lower surface (24, 124); each side abutment surface of thethird pair of side abutment surfaces (62) converges towards each otherin a direction toward the upper surface (22, 122); and each sideabutment surface of the fourth pair of side abutment surfaces (64)converges towards each other in a direction toward the lower surface(24, 124).
 15. The cutting insert (18, 118) according to claim 14,wherein: the third pair of side abutment surfaces (62) has 180°rotational symmetry with the fourth pair of side abutment surfaces (64)around the symmetry axis (S).
 16. The cutting insert (18, 118) accordingto claim 15, wherein: the first pair of side abutment surfaces (58)converge towards each other in a direction toward the upper surface (22,122) as viewed in a cross-section taken in a first section plane (P3),the first section plane contains the through bore axis (B) and isobliquely disposed, as seen in an end view of the cutting insert, withrespect to a symmetry plane (SP) containing the through bore axis (B)and the symmetry axis (S).
 17. The cutting insert (18, 118) according toclaim 16, wherein: the second pair of side abutment surfaces (60)converge towards each other in a direction toward the lower surface (24,124) as viewed in a cross-section taken in a second section plane (P4),the second section plane (P4) contains the through bore axis (B) and isobliquely disposed at a plane angle (φ), as seen in a top view of thecutting insert, with respect to the symmetry plane (SP) and with respectto the first section plane (P3).
 18. The cutting insert (18, 118)according to claim 17, wherein: the plane angle (φ) is 80°.
 19. Thecutting insert (18, 118) according to claim 17, wherein: the peripheralsurface (26) forms with the first reference plane (P1) and with thesecond reference plane (P2) an obtuse first internal included angle (α),as seen in a first side view of the cutting insert that is perpendicularto a secondary cutting edge (44, 50, 144, 150); and the peripheralsurface (26) forms with the first reference plane (P1) and with thesecond reference plane (P2) an acute second internal included angle (β),as seen in a second side view of the cutting insert that isperpendicular to the first side view.
 20. A cutting insert (218) forretention in a rotating cutting tool (10) having an axis of rotation(A), the cutting insert comprising: two opposing end surfaces (228) anda peripheral surface (226) extending therebetween, each end surfacehaving a mutual first axis of symmetry (B) passing through the endsurfaces about which each end surface has 180° rotational symmetry; aperipheral cutting edge (266) formed at the junction between a first endsurface, constituting an upper surface (222), and the peripheralsurface, the peripheral cutting edge comprising two curved cutting edges(232, 234) merging with two straight cutting edges which extend betweenthe curved cutting edges at extremities thereof; a continuouslyextending rake surface (268) extends inwardly from the peripheralcutting edge, the rake surface is slanted at a rake slant angle (λ) withrespect to a second end surface, constituting a lower surface (224); theupper surface comprises a rake inner extremity (92) at the innermostextremity of the rake surface and a bore upper end (96) at the uppermostend of a through bore (30) that extends between the end surfaces, alength between a given point (98) on the peripheral cutting edge and therake inner extremity comprises a first rake length (H1) and a lengthbetween the given point and the bore upper end comprises a second rakelength (H2), the first rake length and the second rake length taken in aplane parallel to the lower surface; wherein: the rake slant angle isequal to or greater than 25°; a rake extension ratio (E), defined as aratio between the first rake length (H1) and the second rake length(H2), is smaller than 1 and equal to or greater than 0.8; and a majorangular portion (MP11, MP12) of each of the curved cutting edges lies ona torus and extends along an angle equal to or larger than 120° as seenalong the first axis of symmetry (B).
 21. The cutting insert (218)according to claim 20, wherein the peripheral surface comprises: a first(258) pair of side abutment surfaces that converge towards each other ina direction toward the upper surface; and a third (262) pair of sideabutment surfaces that converge towards each other in a direction towardthe upper surface.
 22. A cutting tool (10) having a longitudinal axis ofrotation (A) and comprising: a tool body (12) having at least one insertpocket (14) formed in a front end (16) of the tool body and a cuttinginsert (18, 118) retained in the at least one insert pocket, wherein:the at least one insert pocket comprises: a pocket base abutment surface(74); a threaded bore (76) extending tangentially rearwardly from thepocket base abutment surface; pocket side walls (78) extending upwardlyfrom the pocket base abutment surface and comprising a first pocketabutment surface (80) forming an acute first pocket internal angle (γ)with the pocket base abutment surface, and a second pocket abutmentsurface (82) forming an acute second pocket internal angle (δ) with thepocket base abutment surface, the first and second pocket abutmentsurfaces being spaced apart from one another; and the cutting insert(18, 118) comprises: two opposing end surfaces (28, 128) and aperipheral surface (26) extending therebetween, each end surface havinga mutual first axis of symmetry (B) passing through the end surfacesabout which each end surface has N-fold rotational symmetry for somevalue of N where N is a number from the group consisting of 2, 3 and 4;a peripheral cutting edge (66, 166) formed at the junction between eachend surface and the peripheral surface, the peripheral cutting edgecomprising N curved cutting edges (32, 34; 132, 134) merging with Nstraight cutting edges (44, 50; 144, 150) which extend between thecurved cutting edges at extremities thereof, wherein the curved cuttingedges of the two end surfaces are not aligned, as seen in an end view ofthe cutting insert.
 23. The cutting tool according to claim 22, wherein:the cutting insert further comprises first (58), second (60), third (62)and fourth (64) pairs of side abutment surfaces, each pair of the sideabutment surfaces having 180° rotational symmetry around the throughbore axis (B), wherein: the first pair of side abutment surfaces (58)converge towards each other in a direction toward the upper surface (22,122), the second pair of side abutment surfaces (60) converge towardseach other in a direction toward the lower surface (24, 124), the thirdpair of side abutment surfaces (62) converge towards each other in adirection toward the upper surface (22, 122), the fourth pair of sideabutment surfaces (64) converge towards each other in a direction towardthe lower surface (24, 124), the third pair has 180° rotational symmetrywith the fourth pair around a second symmetry axis (S) that passesbetween the third pair and the fourth pair, the upper surface defines afirst reference plane (P1) and the lower surface defines a secondreference plane (P2), the first and second reference planes are parallelto a median plane (M) that is located midway between the upper surfaceand the lower surface; the peripheral surface (26) forms with the firstreference plane (P1) and with the second reference plane (P2) an obtusefirst internal included angle (a), as seen in a first side view of thecutting insert that is perpendicular to a given secondary cutting edge;the peripheral surface (26) forms with the first reference plane (P1)and with the second reference plane (P2) an acute second internalincluded angle (β), as seen in a second side view of the cutting insertthat is perpendicular to the first side view: the upper surface (22,122) is provided with a planar upper central abutment surface (70, 170),constituting an insert upper base abutment surface, that extendsinwardly from an associated rake surface (68, 168) toward the throughbore (30); and the lower surface (24, 124) is provided with a planarlower central abutment surface (72), constituting an insert lower baseabutment surface, that extends inwardly from the associated rake surface(68, 168) toward the through bore (30); wherein, in a retained positionof the cutting insert (18, 118): the insert lower base abutment surface(72) abuts the pocket base abutment surface (74); one abutment surfaceof the insert first pair of side abutment surfaces (58) abuts the firstpocket abutment surface (80); one abutment surface of the insert thirdpair of side abutment surfaces (62) abuts the second pocket abutmentsurface (82); and a clamping screw (20) passes through the through bore(30) of the cutting insert and threadingly engages the threaded bore(76) of the insert pocket.
 24. The cutting tool (10) according to claim23, wherein: the at least one insert pocket (14) is provided with apocket abutment relief surface (84) that is located above the secondpocket abutment surface (82), the pocket abutment relief surface beingrelieved from the adjacent abutment surface of the insert fourth pair ofside abutment surfaces (64) in a retained position of the cuttinginsert.
 25. The cutting tool (10) according to claim 22, wherein: thecutting insert (18, 118) is four times indexable within the at least oneinsert pocket (14).
 26. The cutting tool (10) according to claim 22,wherein: the first pocket abutment surface (80) forms with the secondpocket abutment surface (82) an acute pocket angle (θ) as seen in a topview of the at least one insert pocket (14).
 27. The cutting tool (10)according to claim 22, wherein: the at least one insert pocket (14) isprovided with a pocket relief channel (86) located between the pocketside walls (78) and the pocket base abutment surface (74).
 28. Adouble-sided cutting insert for retention in a rotating cutting toolhaving an axis of rotation, the cutting insert comprising: two opposingend surfaces and a peripheral surface extending therebetween, each endsurface having a mutual first axis of symmetry (B) passing through theend surfaces about which each end surface has N-fold rotationalsymmetry, where N is a number from the group consisting of 2 and 4; aperipheral cutting edge formed at the junction between each end surfaceand the peripheral surface, the peripheral cutting edge comprising Ncurved cutting edges merging with N straight cutting edges which extendbetween the curved cutting edges at extremities thereof; wherein: thecurved cutting edges of the two end surfaces are not aligned, in adirection along the first axis of symmetry (B); the cutting insertcomprises a through bore, having a through bore axis (B) constitutingthe first axis of symmetry, that extends between the two end surfaces;and the peripheral surface comprises first, second, third and fourthpairs of side abutment surfaces, each pair of side abutment surfaceshaving 180° rotational symmetry around the through bore axis (B). 29.The cutting insert according to claim 28, wherein: in a top view of thecutting insert, the curved cutting edges are longer than the straightcutting edges.
 30. The cutting insert according to claim 29, wherein:N=4 and the cutting insert has eight curved cutting edges.
 31. Thecutting insert according to claim 29, wherein: one extremity of anygiven curved cutting edge is located further from a median plane of thecutting insert than the other extremity of the given curved cuttingedge, the median plane being located midway between the end surfaces.32. The cutting insert according to claim 31, wherein: the curvedcutting edges associated with one of the end surfaces define a firstreference plane and the curved cutting edges associated with the otherend surface define a second reference plane, with the entire cuttinginsert being located between the first reference plane and the secondreference plane; and the first and second reference planes are parallelto each other and located equidistant from and on either side of themedian plane of the cutting insert.
 33. The cutting insert according toclaim 32, wherein: at each end surface, the N curved cutting edges andthe N straight cutting edges form a continuously extending peripheralcutting edge that is associated with a rake surface that extendscontinuously along the entire circumference of that end surface; therake surface merges with a central abutment surface; and the entirecentral abutment surface is located closer to the median plane than thereference plane associated with that end surface.
 34. The cutting insertaccording to claim 33, wherein: at said each end surface, the rakesurface merges with the upper central abutment surface through anundercut.
 35. The cutting insert according to claim 32, wherein: thecutting insert has 180° rotational symmetry around a second symmetryaxis (S), the second symmetry axis lies on the median plane between thefirst and the second reference planes and intersects the peripheralsurface at two median insert symmetry points.
 36. The cutting insertaccording to claim 29, wherein: the curved cutting edges associated withone of the end surfaces are located in a first reference plane and thecurved cutting edges associated with the other end surface are locatedin a second reference plane, with the entire cutting insert beinglocated between the first reference plane and the second referenceplane; and the first and second reference planes are parallel to eachother and located equidistant from and on either side of a median planeof the cutting insert, the median plane being located midway between theend surfaces.
 37. The cutting insert according to claim 36, wherein: ateach end surface, the N curved cutting edges and the N straight cuttingedges form a continuously extending peripheral cutting edge that isassociated with a rake surface that extends continuously along theentire circumference of that end surface; the rake surface merges with acentral abutment surface; and the entire central abutment surface islocated closer to the median plane than the reference plane associatedwith that end surface.
 38. The cutting insert according to claim 37,wherein: at said each end surface, the rake surface merges with theupper central abutment surface through an undercut.
 39. The cuttinginsert according to claim 36, wherein: the cutting insert has 180°rotational symmetry around a second symmetry axis (S), the secondsymmetry axis lies on the median plane between the first and the secondreference planes and intersects the peripheral surface at two medianinsert symmetry points.
 40. A cutting tool having a longitudinal axis ofrotation (A) and comprising: a tool body having at least one insertpocket formed in a front end of the tool body and a cutting insert inaccordance with claim 28 retained in the at least one insert pocket,wherein: the at least one insert pocket comprises: a pocket baseabutment surface; and a threaded bore extending tangentially rearwardlyfrom the pocket base abutment surface.
 41. The cutting insert accordingto claim 28, wherein: a section plane containing the through bore axis(B) and any point on any one of the straight cutting edges intersects anadjacent side abutment surface which slopes inwardly with respect to thethrough bore axis (B), in an axial direction away from an associated endsurface and towards the opposite end surface.
 42. A double-sided cuttinginsert for retention in a rotating cutting tool having an axis ofrotation, the cutting insert comprising: two opposing end surfaces and aperipheral surface extending therebetween, each end surface having amutual first axis of symmetry (B) passing through the end surfaces aboutwhich each end surface has N-fold rotational symmetry, where N is anumber from the group consisting of 2 and 4; a peripheral cutting edgeformed at the junction between each end surface and the peripheralsurface, the peripheral cutting edge comprising N curved cutting edgesmerging with N straight cutting edges which extend between the curvedcutting edges at extremities thereof; wherein: in a top view of thecutting insert, the curved cutting edges are longer than the straightcutting edges; a median plane of the cutting insert is located midwaybetween the end surfaces; one extremity of any given curved cutting edgeis located further from the median plane than the other extremity of thegiven curved cutting edge; the cutting insert comprises a through bore,having a through bore axis (B) constituting the first axis of symmetry,that extends between the two end surfaces; and the peripheral surfacecomprises first, second, third and fourth pairs of side abutmentsurfaces, each pair of side abutment surfaces having 180° rotationalsymmetry around the through bore axis (B).
 43. The cutting insertaccording to claim 42, wherein: N=4 and the cutting insert has eightcurved cutting edges.
 44. The cutting insert according to claim 42,wherein: the curved cutting edges associated with one of the endsurfaces define a first reference plane and the curved cutting edgesassociated with the other end surface define a second reference plane,with the entire cutting insert being located between the first referenceplane and the second reference plane; and the first and second referenceplanes are parallel to each other and located equidistant from and oneither side of the median plane of the cutting insert.
 45. The cuttinginsert according to claim 44, wherein: at each end surface, the N curvedcutting edges and the N straight cutting edges form a continuouslyextending peripheral cutting edge that is associated with a rake surfacethat extends continuously along the entire circumference of that endsurface; the rake surface merges with a central abutment surface; andthe entire central abutment surface is located closer to the medianplane than the reference plane associated with that end surface.
 46. Thecutting insert according to claim 45, wherein: at said each end surface,the rake surface merges with the upper central abutment surface throughan undercut.
 47. The cutting insert according to claim 42, wherein: thecutting insert has 180° rotational symmetry around a second symmetryaxis (S), the second symmetry axis lies on the median plane between thefirst and the second reference planes and intersects the peripheralsurface at two median insert symmetry points.
 48. The cutting insertaccording to claim 42, wherein: a section plane containing the throughbore axis (B) and any point on any one of the straight cutting edgesintersects an adjacent side abutment surface which slopes inwardly withrespect to the through bore axis (B), in an axial direction away from anassociated end surface and towards the opposite end surface.